This invention relates to semiconductor laser sources and, more particularly, to semiconductor laser sources that are useful in communication systems.
An important quality of elements that are used in a communication system is the lifetime of the component. In systems such as submarine cable transmission systems, the lifetime of the components may dictate whether or not the communication system is economically feasible relative to other types of communications. One impediment to the immediate use of optical components in a submarine cable system has been the lifetime of the semiconductor laser. In order to achieve the wavelength at which an optical fiber exhibits its lowest transmission loss, the semiconductor laser should be constructed of materials from the indium, gallium, arsenic, phosphorous material system. Unfortunately, the lifetime of these lasers is not sufficiently long as yet to warrant their use in a submarine cable system.
In order to minimize the amount of delay that exists between the electrical pulse and the light pulse that corresponds to the electrical pulse, the semiconductor laser is usually biased at a threshold level at which the laser output begins to sharply increase. This threshold level is not only a function of the individual laser but is also a function of both temperature and time. In order to ensure that the laser is always properly biased at a correct threshold level, circuits in the prior art have been proposed and utilized to control the threshold level as a function of the laser output light level. Some of these circuits also control the extinction ratio, that is, the ratio between the peak light intensity output to the low level light intensity output. This latter control of the extinction ratio is also achieved through a utilization of a feedback circuit. Typically both of the feedback circuits receive their input signal from a photodetector which is properly placed to sample the output light from the semiconductor laser. In many cases this photodetector is positioned so as to receive the light emitted from the end of the laser that is opposite to the end that provides the light input to an optical fiber. One such typical circuit can be found in the article entitled "GaAlAs Laser Transmitter For Lightwave Transmission Systems", by P. W. Shumate, Jr. et al, Bell System Technical Journal Vol. 57, No. 6 (July-August 1978) pp. 1823-1836.
The prior art has also recognized that a multilaser source package can be placed on silicon which serves as a substrate for both the laser array and other optical components. See, for example, the digest entitled "GaAs Laser Source Package for Multichannel Optical Links", by J. D. Crow et al, pp. WB6-1-WB6-3 in the Optical Fiber Transmission II Technical Digest, Williamsburg, Virginia, Feb. 22-24, 1977. In the Crow et al talk, it was pointed out that this silicon substrate can also serve as the substrate for the individual laser drive electrodes and additional electronic circuitry that may be used to drive the multilaser source package. The lasers in the Crow et al apparatus appear, however, to function simultaneously with each laser delivering its output to an individual fiber light guide. There is no suggestion as to how this laser array can be utilized to provide a laser source with an extended life for use in systems such as submarine cable transmission systems.